There are several possibilities for Blackstone bracelet. First of all, the stone itself may contain some substances, which are polluted or oxidized during long-term use, leading to blackening. Secondly, the bracelet may be affected by pollutants, such as dust, chemicals, sweat, cosmetics and so on. These substances will react with oxygen in the air and make the stone surface black. In addition, if the bracelet is a low-quality stone, it may turn black because of high impurities and moisture content.

In order to prevent the stone bracelet from blackening, the following measures can be taken:

1. Regular cleaning: gently clean the bracelet with mild soapy water or jewelry cleaner, and then dry it with a soft cloth.

2. Avoid pollution: try to avoid contact between the bracelet and chemicals, liquids and sweat, so as not to accelerate the process of oxidation and blackening.

3. Storage method: When storing bracelets, be careful not to be exposed to air for a long time. You can put the bracelet in a dry place or pack it in a sealed bag.

If the bracelet has turned black, you can try to use some household cleaners and cleaning methods to restore its original brightness. However, some bracelet materials, such as natural stones or precious stones, may need professional maintenance to maintain their appearance. If the problem of bracelet blackening cannot be solved, it is recommended to consult the staff of professional jewelry stores or stone jewelry stores for further consultation.